Infrared and blue stacked laser diode array by wafer fusion

ABSTRACT

An an infrared laser structure has an inverted or p-side down orientation. The infrared laser structure is inverted and wafer fused to a blue laser structure to form an infrared/blue monolithic laser structure. The top semiconductor layer of the inverted infrared stack laser structure is a GaInP fusion bonding layer which will be wafer fused to the top semiconductor layer of the blue laser structure which is a GaN cladding/contact layer.

BACKGROUND OF THE INVENTION

This invention relates to a monolithic integrated infrared and bluewavelength laser structure and, more particularly, to an IR laserstructure which is wafer fused to a blue laser structure.

Independently addressable monolithic dual wavelength light sources,especially arrays that can simultaneously emit two different wavelengthlight beams from two different laser elements in the monolithicstructure are useful in a variety of applications, such as colorprinting, full color digital film recording, color displays, and otheroptical recording and storage system applications.

The performance of many devices, such as laser printers and opticalmemories, can be improved by the incorporation of dual laser beams. Forexample, laser printers which us dual beams can have higher printingspeeds and/or better spot acuity than printers which use only a singlebeam. Recent advances in xerography, such as described in commonlyassigned Kovacs et al. U.S. Pat. No. 5,347,303 on "Full ColorXerographic Printing System with Dual Wavelength, Single Optical SystemROS and Dual Layer Photoreceptor" (which is hereby incorporated byreference), have created quad-level xerography (sometimes referred to as"xerocolography") that enables the printing of three colors (forexample, black plus two highlight colors) in a single pass by a singlexerographic station.

In these and many applications, closely spaced laser beams of twodifferent wavelengths are desirable.

One way to obtain closely spaced, dual wavelength laser beams is to formdual laser emission sites, or laser stripes, on a common substrate.While this enables very closely spaced beams, prior art monolithic laserarrays typically output laser beams at only one wavelength.

Various techniques are known in the prior art for producing twodifferent wavelength laser beams from a monolithic laser array. Forexample, it is well known that a small amount of wavelength differencecan be obtained by varying the drive conditions at each of the twolasing regions. However, the easily achievable but small wavelengthdifference is insufficient for most applications.

Ideally, for most desired applications, the laser elements should emitlight of different widely spaced wavelengths. In a preferred monolithicstructure, the laser elements would emit light across a widely spacedspectrum from infrared to blue wavelengths. One problem is that lasersources of different wavelengths require different light emission activelayers; i.e. nitride semiconductor layers such as InGaN for blue lasersand arsenide semiconductor layers such as AlInGaAs for infrared lasers.

One method of achieving these larger wavelength separations is to grow afirst set of active layers on a substrate to form a first lasing elementwhich outputs light at one wavelength, and then to etch and regrow asecond set of active layers next to the first to form a second lasingelement at a second wavelength. However, this method requires separatecrystal growths for each lasing element, something which is not easilyperformed. Furthermore, the arsenide semiconductor structures ofinfrared lasers use a different, non-compatible substrate with thenitride semiconductor structures of blue lasers. Lattice mismatchingbetween semiconductor layers will result in poor or non-existentperformance of one or more of the laser structures.

Another technique for obtaining different wavelength laser beams from amonolithic laser array is to use stacked active regions. A stackedactive region monolithic array is one in which a plurality of activeregions are sandwiched between common cladding layers. Each activeregion is comprised of a thin volume that is contained within a laserstripe. The laser stripes contain two different numbers of activeregions that emit laser beams at two different wavelengths.

In a stacked active region monolithic laser array, current flows inseries through the stacked active regions. The active region with thelowest bandgap energy will lase, thereby determining the wavelength ofthe laser beam output from that part of the array. To provide anotherwavelength output, the previously lowest bandgap energy active region isremoved from part of the array and current is sent through the remainingstacked regions.

A major problem with stacked active region monolithic laser arrays isthat they have been difficult to fabricate, even with just arsenide andphosphide semiconductor layers. The addition of nitride semiconductorlayers makes optical performance nearly impossible and impractical inany real world applications.

It is an object of this invention to provide stacked active regionlasers in a monolithic structure capable of outputting closely spaced,multiple wavelength laser beams in the infrared to blue wavelengthspectrum.

SUMMARY OF THE INVENTION

The present invention provides an infrared laser structure having aninverted or p-side down orientation. The infrared laser structure isinverted and wafer fused to a blue laser structure to form aninfrared/blue monolithic laser structure. The top semiconductor layer ofthe inverted infrared stack laser structure is a GaInP fusion bondinglayer which will be wafer fused to the top semiconductor layer of theblue laser structure which is a GaN cladding/contact layer.

Other objects and attainments together with a fuller understanding ofthe invention will become apparent and appreciated by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of the semiconductor layers of theIR inverted laser structure of the present invention.

FIG. 2 is a cross-sectional side view of the semiconductor layers of theBlue laser structure of the present invention.

FIG. 3 is a cross-sectional side view of the semiconductor layers of theIR/Blue stack laser structure formed by wafer fusion of the presentinvention.

FIG. 4 a cross-sectional side view of the IR/Blue stack laser structurewith independently addressable contacts for each laser structure andwith a metal clad ridge waveguide of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention involves fabricating an IR laser structure with aninverted or p-side down orientation, fabricating a Blue laser structurewith standard or p-side up orientation, flipping over the inverted IRstack laser structure to wafer fuse the IR laser structure to the Bluelaser structure and forming the resulting IR/Blue stack lasermonolithically integrated structure with standard or p-side uporientation for each laser.

Reference is now made to FIG. 1 which illustrates an IR inverted laserstructure 100 in accordance to the present invention. The laserstructure 100 is grown in an inverted or p-side down orientation.

As shown in FIG. 1, a substrate removal etch stop layer 104 of p-Ga₀.5In₀.5 P of 0.2 μm thickness is grown on a GaAs substrate 102 using awell-known epitaxial deposition process commonly referred to asmetal-organic chemical vapor deposition (MOCVD). Other depositionprocesses such as liquid phase epitaxy (LPE), molecular beam epitaxy(MBE), or other known crystal growth processes can also be used. Thep-GaInP etch stop layer 104 has a magnesium doping of 1×10¹⁸ cm⁻³. TheGaAs substrate 102 can be p-type or n-type or semi-insulating.

Upon the substrate removal etch stop layer 104 is deposited a p+-GaAscap layer 106, which typically is 100 nanometers thick with a magnesiumdoping of 1×10¹⁹ cm⁻³. Upon the p+-GaAs cap layer 106 is deposited abarrier reduction p-type Gas₀.5 In₀.5 P layer 108, which typically has athickness of 50 nanometers and a magnesium doping level of approximately5×10¹⁸ cm⁻³.

Upon the GaInP barrier reduction layer 108 is deposited a p-type Al₀.5In₀.5 P lower cladding layer 110. Typically, this p-cladding layer 110has an aluminum content of 50% and a magnesium doping level of 1×10¹⁸cm⁻³. The thickness of the AlInP cladding layer 110 is approximately onemicron (μm).

Above the lower p-cladding layer 110 is deposited an undoped Al₀.4 Ga₀.6As lower confinement layer 112, which has a thickness of about 120nanometers. After this lower confinement layer 112 has been deposited,an Al₀.5 Ga₀.7 In₀.15 As active layer 114 is deposited, which shouldresult in a light emission at approximately 820 nanometers. The activelayer 114 may be a single quantum well, a multiple quantum well, or alayer with thickness greater than that of a quantum well. The thicknessof a quantum well typically ranges from five to twenty nanometers and inthis example is 7 nanometers. Above the active layer 114 is deposited anundoped Al₀.4 Ga₀.5 As upper confinement layer 116. The confinementlayer 116 has a thickness of about 120 nanometers. The lower and upperconfinement layers, 112 and 116, together with the active layer 114,form the active region 117 of the laser structure.

After the upper confinement layer 116 has been formed, a n-type Al₀.5In₀.5 P upper cladding layer 118 of about one micron is deposited. Thedoping level of the upper cladding layer is 1×10¹⁸ cm⁻³.

Upon the upper cladding layer 118 is deposited a n-type GaAs contactlayer 120. The thickness of the GaAs layer 120 is approximately one-halfmicron (0.5 μm). The doping level of the n-type GaAs layer 120 isapproximately 5×10¹⁸ cm⁻³.

These semiconductor layers 106 to 120 form the infrared laser structure122.

Upon the n-GaAs layer 120 of the infrared laser structure 122 isdeposited a n-type Ga₀.5 In₀.5 P fusion bonding layer 124, whichtypically has a thickness of 50 nanometers and a silicon doping level ofapproximately 5×10¹⁸ cm⁻³. Upon the n-GaInP fusion bonding layer 124 isdeposited a n+-GaAs protective cap layer 126, which typically is 100nanometers thick with a silicon doping of 1×10¹⁹ cm⁻³.

Reference is now made to FIG. 2 which illustrates a Blue laser structure200 in accordance to the present invention. The laser structure 200 isgrown in a standard or p-side up orientation.

The Blue laser structure 200 has a sapphire (Al₂ O₃) substrate 202. Uponthe substrate 200 is deposited an undoped GaN buffer layer 204 which hasa thickness of 30 nanometers. A n-GaN cladding layer 206 is deposited onthe undoped buffer layer 204. The cladding layer is silicon doped at1×10¹⁸ cm⁻³ and has a thickness of 4 microns. A n-In₀.5 Ga₀.95 stressreduction stress reduction layer 208 is deposited on the n-GaAs claddinglayer 206. The InGaN stress reduction layer 208 has a thickness of 0.1microns and a silicon doping level of 1×10¹⁸ cm⁻³. Upon the InGaN stressreduction layer 208 is deposited a n-Al₀.08 Ga₀.92 N lower confinementlayer 210. The n-confinement layer 210 is silicon doped at 1×10¹⁸ cm⁻³and has a thickness of 0.5 microns.

An n-GaN waveguiding layer 212 is deposited on the n-AlGaN lowerconfinement later 208. The GaN waveguiding layer 212 is 0.1 micronsthick and is silicon doped at 1×10¹⁸ cm⁻³. An In₀.15 Ga₀.85 N/In₀.02Ga₀.98 N multiple quantum well active layer 214 is deposited on thewaveguiding layer 212. The In₀.15 Ga₀.85 N/In₀.02 Ga₀.98 N multiplequantum well active layer 214 has 3 to 20 quantum wells and is about 50nanometers thick and emits light at 410 to 430 nanometers. A p-Al₀.2Ga₀.8 N carrier confinement layer 216 is deposited on the active layer214. The p-AlGaN carrier confinement layer 216 is magnesium doped at5×10¹⁹ cm⁻³ and has a thickness of 0.02 microns. An p-GaN waveguidinglayer 218 is deposited on the p-AlGaN carrier confinement later 216. TheGaN waveguiding layer 218 is 0.1 microns thick and is magnesium doped at5×10¹⁹ cm⁻³. The waveguiding layers 212 and 218, together with theconfinement layer 216 and the active layer 214 form the active region219 of the laser structure.

A p-Al₀.08 Ga₀.92 N upper confinement layer 220 is deposited on thewaveguiding layer 218. The p-confinement layer 220 is magnesium doped to5×10¹⁹ cm⁻³ and has a thickness of 0.5 microns. A p-GaN cladding/contactlayer 222 is deposited on the p-confinement layer 220. Thecladding/contact layer 222 is magnesium doped to 5×10¹⁹ cm⁻³ and has athickness of 0.5 microns.

The exposed upper surface 128 of the cap layer 126 of the IR laserstructure 100 of FIG. 1 and the exposed upper surface 224 of thecladding/contact layer 222 of the Blue laser structure 200 of FIG. 2 arecleaned with solvents. The exposed surface 224 of the Blue laserstructure 200 is immersed in hydroflouric acid (HF). The exposed surface128 of the layer 126 is immersed in a solution of sulfuric acid:hydrogenperoxide:water (H₂ SO₄ :H₂ O₂ :H₂ O). This solution selectively etchesthe GaAs protective capping layer 126, exposing the surface 130 of thefusion bonding layer 126. The fusion bonding layer is etched for 1minute in hydrobromic acid (HBr). The surface 130 is rinsed in deionizedwater. The surface 224 of the Blue laser structure 200 is removed fromthe HF and rinsed in deionized water. Both the Blue laser structuresurface 224 and the IR laser structure surface 130 are blown dry withnitrogen gas. The exposed surface 130 of the fusion bonding layer 126 ofthe IR laser structure 100 is pressed together with the exposed surface224 of the contact/cladding layer 222 of the Blue laser structure 200,as shown in FIG. 3. The joined stack laser structure 300 of FIG. 3 isthen placed into a quartz/graphite fixture (not shown) which appliesuniform, uniaxial pressure on the laser structure 300. The fixture isplaced into a furnace and heated to a temperature of 750° C. for sixtyminutes in a hydrogen gas ambient. Pressure is applied to the laserstructure 300 by utilizing the differential expansion coefficients ofthe quartz and graphite materials in the fixture. The fixture appliespressure in the range of 1 to 8 MPa. The fusion bonding layer 126 of theIR stack laser structure 100 is thus fused along an interface 302 to thecladding/contact layer 222 of the Blue laser structure forming anIR/Blue stack laser structure 300.

The sacrifical GaAs substrate 102, which has provided structural supportduring the deposition of the red laser structure and during thesubsequent wafer fusion of the red laser structure to the blue laserstructure, is removed by chemical/mechanical polishing with bleach to athickness of 50 to 100 microns. The remainder of the GaAs substrate 102is completely etched away by a sulfuric acid: hydrogen peroxide (H₂ SO₄:H₂ 0₂) solution to the substrate removal etch stop layer 104. Thesubstrate removal etch stop layer 104 is removed by etching withhydrobromic acid (HBr) to the cap layer 106. The p+GaAs cap layer 106 ofthe red laser structure is now the uppermost semiconductor layer in theIR/Blue stack laser structure 300.

Wafer fusion creates a non-lattice matched heterostructure, themonolithically integrated laser structure 300 in this embodiment. Thismonolithic integration of the IR/Blue stack laser structure 300 providesa closely spaced, precisely spaced structure of two different, widelyspaced wavelength laser sources, necessary for precision opticalsystems.

The IR laser structure 100 will be inverted to standard p-side uporientation then fused to the blue laser structure 200 which was alreadyin the standard p-side up orientation. The resulting IR/Blue stack laserstructure 300 has a standard p-side up orientation.

As shown in FIG. 3, the resulting IR/Blue stack laser structure 300after wafer fusion has semiconductor layers, in sequence, of a sapphire(Al₂ O₃) substrate 202, an undoped GaN buffer layer 204, a n-GaNcladding layer 206, a n-In₀.25 Ga₀.95 N stress reduction layer 208, an-Al₀.08 Ga₀.92 N lower confinement layer 210, an n-GaN waveguidinglayer 212, an In₀.15 Ga₀.85 N/In₀.02 Ga₀.98 N multiple quantum wellactive layer 214, a p-Al₀.2 Ga₀.8 N carrier confinement layer 216, ap-GaN waveguiding layer 218, a p-Al₀.08 Ga₀.92 N upper confinement layer220, a p-GaN cladding/contact layer 222, a fusion bonding layer 124 ofGaInP, a n-type GaAs contact layer 120, a n-type Al₀.5 In₀.5 P uppercladding layer 118, an undoped Al₀.4 Ga₀.6 As confinement layer 116, anGaAs active layer 114, an undoped Al₀.4 Ga₀.6 As confinement layer 112,a p-type Al₀.5 In₀.5 P cladding layer 110, a barrier reduction p-Ga₀.5In₀.5 P layer 108, and a p+-GaAs cap layer 106.

The semiconductor layers 202 to 222 form the blue laser structure 200.The semiconductor layers 106 to 120 form the infrared laser structure122. From the bottom up, the IR/Blue stack laser structure 300 has ablue laser structure 200 and a infrared laser structure 122. Theinfrared laser structure 122 now has standard p-side up orientation.

As shown in FIG. 4, independently addressable infrared and blue laserscan be fabricated in a monolithic laser array structure by conventionalmasking and etching and conventional deposition of metal contacts. Forease of understanding, the masking steps are not shown and the etchingand contact deposition are not shown in order.

In the infrared laser structure 122 in FIG. 4, a portion 304 is etcheddown through the cap layer 106, the barrier reduction layer 108, thep-cladding layer 110, the confinement layer 112, the active layer 114,the confinement layer 116 and the n-cladding layer 118 to the n-contactlayer 120. An Au:Ge n-contact 306 is formed on the surface 308 of then-contact layer 120 for the infrared laser structure 122.

In the infrared laser structure 122 in FIG. 4, portions 310 are etchedthrough the cap layer 106, the barrier reduction layer 108, and thep-cladding layer 110 to the confinement layer 112 forming a mesastructure. A Ti-Au p-contact 312 is formed on the surface 314 of theconfinement layer 118 and the unetched cap layer 106, the barrierreduction layer 108 and the p-cladding layer 110 for the infrared laserstructure 122. The remaining unetched p-cladding layer 110 forms a metalclad ridge waveguide 316 for the infrared laser structure 122.

An isolation groove 317 is etched between the the infrared laserstructure 122 and the blue laser structure 200 down to the p-GaNcladding/contact layer 222 of the blue laser structure 200 to provideelectrical and thermal isolation between the two infrared and blue laserstructures in order to reduce crosstalk between the two laserstructures.

In the blue laser structure 200 in FIG. 4, the semiconductor layers ofthe infrared laser structure 122 and fusion bonding layer 124 are etchedaway to the surface 318 of the p-GaN cladding/contact layer 222. A Ti-Aup-contact 320 is formed on the surface 318 of the p-GaN cladding/contactlayer 222 for the blue laser structure 200.

In the blue laser structure 200 of FIG. 4, a portion 322 is etched downthrough the cladding/contact layer 222, the p-confinement layer 220, thewaveguiding layer 218, the p-carrier confinement layer 216, the activelayer 214, the waveguiding layer 212, the confinement layer 210, thestress reduction layer 208 and into the n-cladding layer 206. An Au:Gen-contact 324 is formed on the surface 326 of the n-cladding layer 206for the blue laser structure 200.

The IR/Blue stack laser structure 300 is an edge emitting array.Conventional facets (not shown) are provided on the edge of the laserstructure 300. The facets can be formed by dry etching the IR laserstructure 100 and the Blue laser structure 200. Alternately, if the Bluelaser structure has an a-face sapphire substrate 202, then the IR laserstructure 100 and the Blue laser structure 200 can be cleavedsimultaneously in the IR/Blue stack laser structure 300.

The infrared laser structure 122 will emit light of infrared wavelengthfrom the active region 117 including the active layer 114 through theedge of the laser structure. The infrared laser structure 122 isindependently addressable through contacts 306 and 312 separate from theblue laser structure 200.

Similarly, the blue laser structure 200 will emit light of bluewavelength from the active region 2xx including the active layer 214through the edge of the laser structure. The blue laser structure 200 isindependently addressable through contacts 320 and 324 separate from theinfrared laser structure 122.

The actual steps in fabricating the independently addressable IR/Bluestack laser structure 300 would include silicon nitride masking of theinfrared laser structure 122 and then etching through dry etching orreactive ion etching down to the blue laser structure 200, masking ofthe infrared laser structure 122 then etching the isolation groove,masking of the mesa in the infrared laser structure 122 then etching tothe n-cladding layer, masking of the mesa and the n-cladding layer ofthe infrared laser structure 122 and a portion of the blue laserstructure 200 then etching down to the n-cladding layer of the bluelaser structure 200, formation of the p-contacts on the mesa of theinfrared laser structure 122 and formation of the p and n contacts ofthe blue laser structure 200, masking of the blue laser structure 200and the p-contact of the infrared laser structure 122 then etching tothe n-cladding layer of the infrared laser structure 122, and formationof the n-contact on the n-cladding layer of the infrared laser structure122.

The use of a mesa laser structure and metal clad ridge waveguide for theinfrared laser structure is merely an illustrative example. The upperconfinement layer of p-AlInP can form a native oxide ridge waveguide.The specific cladding, confinement and active layers for the infraredand blue laser structures can be fabricated from different semiconductormaterials other than those listed in this embodiment.

The fusion bonding layer 124 could, in the alternative, be anotherindium containing layer such as non-lattice matched InP.

The composition, dopants, doping levels, and dimensions given above areexemplary only, and variations in these parameters are permissible.Additionally, other layers in addition to the ones shown in the figuresmay also be included. Variations in experimental conditions such astemperature and time are also permitted.

While the invention has been described in conjunction with specificembodiments, it is evident to those skilled in the art that manyalternatives, modifications, and variations will be apparent in light ofthe foregoing description. Accordingly, the invention is intended toembrace all such alternatives, modifications, and variations that fallwithin the spirit and scope of the appended claims.

What is claimed is:
 1. A monolithic integrated edge-emittingsemiconductor laser structure comprising:a first laser structure havingafirst substrate; an n-type first cladding layer formed on saidsubstrate; a first confinement layer, a first active layer for emittinglight of a first wavelength, and a second confinement layer forming afirst active region on said first cladding layer; a p-type secondcladding layer formed on said second confinement layer above said firstactive region, a second laser structure havinga fusion layer for bondingsaid first laser structure and said second laser structure, an n-typefirst contact layer formed on said fusion layer; an n-type thirdcladding layer formed on said first contact layer; a third confinementlayer, a second active layer for emitting light of a second wavelength,and a fourth confinement layer forming a second active region on saidthird cladding layer; a p-type fourth cladding layer formed on saidfourth confinement layer above said second active region, a p-typesecond contact layer formed on said fourth cladding layer, a firstcontact and a second contact which enable biasing of said first activeregion for emission of light of said first wavelength, and a thirdcontact and a fourth contact which enable biasing of said second activeregion for emission of light of said second wavelength.
 2. Themonolithic integrated edge-emitting semiconductor laser structure ofclaim 1 wherein said first wavelength is in the blue range and saidsecond wavelength is in the infrared range.
 3. The monolithic integratededge-emitting semiconductor laser structure of claim 2 wherein saidfirst substrate of said first laser structure is Al₂ O₃, said first andsecond cladding layers of said first laser structure are GaN, said firstand second confinement layers of said first laser structure are Al₀.08Ga₀.92 N, said first active layer of said first laser structure is anIn₀.15 Ga₀.85 N/In₀.02 Ga₀.98 N multiple quantum well, said fusion layeris GaInP, said first contact layer of said second laser structure isGaAs, said third and fourth cladding layers of said second laserstructure are Al₀.5 In₀.5 P, said third and fourth confinement layers ofsaid second laser structure are Al₀₄ Ga₀.6 As, said second active layerof said second laser structure is GaAs, and said second contact layer ofsaid second laser structure is GaAs.
 4. The monolithic integratededge-emitting semiconductor laser structure of claim 2 wherein saidfirst contact is on said second cladding layer of said first laserstructure, said second contact is on said first cladding layer of saidfirst laser structure, said third contact is on said second contactlayer of said second laser structure, and said fourth contact is on saidthird cladding layer of said second laser structure.